Incorporating fragile additives, such as hollow glass spheres, into unfilled thermoplastic resins without substantial breakage of the spheres presents processing problems not heretofore encountered with traditionally used thermoset resins.
Before curing, thermoset resins have a relatively lower viscosity than thermoplastic resin and thereby provide a more readily workable medium. Generally speaking, the more viscous thermoplastic resins require more work and higher shear forces in order to process before cure. Hollow sphere fracture is undesirable because it raises the specific gravity of the composite and defeats the purpose for adding the spheres. Even further fracturing of the spheres is known to occur during subsequent compression and, in particular, injection molding processes, used to form articles, such as car bumpers. In order to take full advantage of the specific gravity reduction seen in the presence of hollow spheres, the integrity of the spheres must be maintained during compounding and subsequent processing or molding.
It has now been discovered that significant reduction in hollow sphere fracture can be realized by adding the spheres to the thermoplastic resin during the in situ polymerization of the resin. Additionally, a greater volume of spheres than heretofore known can be loaded into the resin by this method.
An in situ polymerized thermoplastic resin incorporating hollow glass spheres by this method, has utility as a masterbatch additive for compounding into other resins, particularly resins filled with fiberglass reinforcement. During the last decade, the automotive and aircraft industries have concentrated their efforts on reducing the overall weight of the vehicles and aircraft. Initially, weight reduction was achieved by replacing metal parts with suitable plastic parts made from thermoset resins. However, in structural applications, the plastic parts are very often lacking in mechanical performance. Thus, to improve their performance, various reinforcing fibrous materials, such as glass fibers, carbon fibers and aramid fibers, are added to the plastic resin. As a cost savings measure, low cost inorganic fillers, such as talc, calcium carbonate or clay, are additionally added to the resin.
However, the addition of the fibers and inorganic fillers undesirably increases the composite's specific gravity and weight per molded part. Accordingly, an increase in price per unit volume may follow. To counteract this weight increase, it is known that hollow glass spheres may be added, but their density lowering effect has not heretofore been fully realized. Substantial sphere fracture occurs during conventional melt compounding with fiber reinforced resins. To solve this problem the inventors add the spheres in the form of a concentrate of spheres dispersed in resin, or a masterbatch, (which is the subject of a copending application).
By using the inventive masterbatch, a significant reduction or elimination of hollow glass sphere fracture during compounding with a resin containing fibrous reinforcements, especially large amounts, will be realized. It is believed that the masterbatch resin covering the glass spheres protects them from breakage by lessening the effects of the shear forces exerted by the mechanical action of the compounding apparatus and the abrasive contact with the glass fibers. Consequently, the use of this inventive masterbatch permits the density-lowering effect of the spheres to be more fully realized in that the lowered density of reinforced resins incorporating the spheres can be better maintained throughout processing and reprocessing.